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Genetically Modified Mosquitoes
Overview Mosquitoes are vectors for several major pathogens such as the Plasmodium species that cause malaria. Controlling mosquito populations in natures faces many obstacles. For example, widespread use of pesticides can damage the environment. In addition, the pesticides are also losing their effectiveness as the mosquitoes become resistant. Sterile Insect Technique A new method to control mosquito populations is to release sterile male mosquitoes into the environment. When these modified males mate with wild type females no offspring are produced. This approach, known as the sterile insect technique, has been used for several other insect pests with positive results (4). The technique uses gamma rays from radioactive cobalt or cesium to irradiate the mosquitoes. Germs cells that produce sperm are more susceptible to radiation damage than regular somatic cells, therefore, low doses of radiation will cause sterility without killing the mosquitoes. This sterile insect technique is potentially advantageous because it is environmentally friendly and should not select for resistance. Best results are seen when only sterile males are released, because sterile females can still transmit infections to people, and because sterile males might try to mate with sterile females instead of normal females which would decrease effectiveness of the technique. The technique poses a major technical challenge since it requires large numbers of male mosquitoes. Although male and female mosquitoes can be separated manually, it is highly inefficient process and requires lots of labor. Novel approach to this problem involves creating male mosquitoes that express a fluorescent protein and then using flow cytometry to automatically separate males from females. Construction of GFP-labeled male mosquitoes The sequence for the green fluorescent protein (GFP) was inserted into a plasmid downstream of the promoter for Beta 2 tubulin which is only expressed in the testes of male mosquitoes. The plasmid also contained the marker dsRed controlled by an actin promoter which is expressed in all cells. This plasmid was micro-injected into mosquito embryos which were then analyzed by flow cytometry. Flow Cytometry One novel technique that can be used to automatically separate males from females is the flow cytometer. The instrument is normally used to analyze and sort cells that are tagged with a fluorescent antibody on the cell surface. The cells then flow through a nozzle one at a time where they pass through a laser that reads the fluorescent tag. A computer registers the fluorescent properties and applies a mild electric charge accordingly. The cell then passes through a charged metal plate where it is directed to its appropriate location. Isolation of GFP-Labeled Male Mosquitoes Flow cytometry can also be used to sort fluorescent tagged mosquito larvae. The flow cytometer was initially used to isolate embryos that were successfully transfected and express the dsRed marker. These embryos were then allowed to hatch and the flow cytometer was used a second time to isolate the male larvae that express the GFP in their testes. These male larvae could then be irradiated and used for the sterile insect technique. References 1. http://www.nature.com/nbt/journal/v23/n11/full/nbt1105-1371.html 2. (img) http://pubs.acs.org/subscribe/archive/mdd/v07/i11/html/1104feature_willis.html 3. http://www.ncbi.nlm.nih.gov/pubmed/16244659 4. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2777328/ ME, Parker AG, Knols BG. Radiation biology of mosquitoes. Malar J. 2009 Nov 16;8 Suppl 2:S6.